With the recent upsurge of the wireless communication market, microwave transistors are playing critical roles in many aspects of human activities.
The requirements for the performance of microwave transistors are becoming more and more demanding. In the personal mobile communication applications, next generation cell phones require wider bandwidth and improved efficiency. The development of satellite communications and TV broadcasting requires amplifiers
operating at higher frequencies (from C band to Ku band, further to Ka band) and higher power to reduce the antenna size of terminal users. The same requirement holds for broadband wireless internet connections as well because of the ever increasing speed or data transmission rate.
Because of these needs, there has been significant investment in the development of high performance microwave transistors and amplifiers based on Si/SiGe, GaAs, SiC and GaN. The Johnson Figure of Merit (JM) gives the power-frequency limit based solely on material properties and can be used to compare different materials for high frequency and high power applications. The requirement for high power and high frequency requires transistors based on semiconductor materials with both large breakdown voltage and high electron velocity. From this point of view, wide bandgap materials, like GaN and SiC, with higher JM are preferable. The wide bandgap results in higher breakdown voltages because the ultimate breakdown field is the field required for band-to-band impact ionization. Moreover, both have high electron saturation velocities, which allow high frequency operation.
The ability of GaN to form heterojunctions makes it superior compared to SiC, in spite of having similar breakdown fields and saturation electron velocities. GaN can be used to fabricate high electron mobility transistors (HEMTs) whereas SiC can only be used to fabricate metal semiconductor field effect transistors (MESFETs). The advantages of the HEMT include its high carrier concentration and its higher electron mobility due to reduced ionized impurity scattering. The combination of high carrier concentration and high electron mobility results in a high current density and a low channel resistance, which are especially important for high frequency operation and power switching applications.
From the amplifier point of view, GaN-based HEMTs have many advantages over existing production technologies. The high output power density allows the fabrication of much smaller size devices with the same output power. Higher impedance due to the smaller size allows for easier and lower loss matching in amplifiers. The operation at high voltage due to its high breakdown electric field not only reduces the need for voltage conversion, but also provides the potential to obtain high efficiency, which is a critical parameter for amplifiers. The wide bandgap also enables it to operate at high temperatures. At the same time, the HEMT offers better noise performance than that of MESFET's. These attractive features in amplifier applications enabled by the superior semiconductor properties make the GaN-based HEMT a very promising candidate for different power applications.
A lot of research has been conducted to improve linearity and normally on or off behavior of the HEMT. See, e.g., U.S. 20080283870. However, there is also a need to further increase the output power of HEMTs.